The aliens are invading?! Relax, Earth has Tom Cruise on its side.

Even if you live on another planet, you won't need a breaking-news update to know that Steven Spielberg's "War of the Worlds" opens Wednesday. The alien-invasion movie's relentless ads have spread the word throughout the galaxy by now.

It's a pretty good movie, too. Tom Cruise gives one of his most intense performances, and the visual effects have enough high-tech power to make an army of interstellar invaders cringe.

The first hour is as good as today's science-fiction thrillers get, building strong scariness with help from two of Mr. Spielberg's regular partners, camera wizard Janusz Kaminski and music composer John Barry.

The film isn't quite excellent, though, since it sags in the middle and starts to seem repetitive. This is partly because it sees everything through the eyes of only three major characters, the feckless father played by Mr. Cruise, and the two children he's determined to protect. This boosts the story's psychological impact, but narrows its range and leads to implausible touches - such as the fact that they're always present when something dramatic happens, but never get harmed like the extras all around them.

Then too, the movie's repertoire of terror stays pretty much the same, from the moment when the first alien "tripod" bursts through a street and starts vaporizing everyone in sight (except our three heroes, of course).

By the time Cruise and company take refuge in the house of a hospitable weirdo, played with oddball charm by Tim Robbins, it's time for the movie to end. But it doesn't, for another 45 minutes or so.

Publicity for the film quotes a grandson of H.G. Wells, who wrote the original "War of the Worlds" novel in 1898, as saying the plot's premise seems especially relevant "each time there is a fear of an invasion" that shakes our sense of security.

Historically, this is hard to argue with. Britain was edgy about Germany when the novel first came out. Orson Welles's infamous radio adaptation - so realistic that many listeners fled their homes in fright - aired in 1938, as world-war fears were building. Hollywood's first version debuted in 1953, amid uncertainties of the postwar period.

So it's not much of a stretch to suspect Spielberg of deliberately tapping into anxieties stirred by the World Trade Center tragedy and the "war on terror" still raging. He implicitly says so in the picture's promotional notes: "I thought now would be a good time to send [this story] crashing down around everybody's ears."

Some may feel that starting the movie's horrors in the greater New York area is a legitimate mood-setting device, while others may see it as tactless exploitation. I think it's some of both.

If one thing is for certain, it's that Spielberg wants to conquer the globe as much as those aliens do, and is a lot better at going about it. Back in 1975, his "Jaws" set the pattern for the modern blockbuster, opening on thousands of American screens after a huge advertising blitz.

His new epic tops this, opening Wednesday "all over the world," as its distributors proclaim. Spielberg's friends and foes must all be rooting for it, hoping for a turnaround in the box-office slump of recent months. I'm rooting for it, too. But only the first hour.